a. Field of the Invention
This invention relates to pollution control equipment used for reducing particulate matter, nitrogen oxides ("NOx"), sulfur dioxide ("SO.sub.2 "), and mercury ("Hg") emissions from the burning of fossil fuels.
b. Description of the Related Art
Electric utilities, and industrial plants typically burn fossil fuels like coal to produce electric power and heat for process requirements. Burning fossil fuel produces an emissions stream containing a number of noxious substances as by-products. These substances include fine particulate matter, mercury and oxides of nitrogen and sulfur. Fine particulate matter has been shown in a recent study to contribute to the early deaths of 64,000 people in the United States alone. Oxides of nitrogen, generally known as NOx, result in the formation of ground level ozone, O.sub.3, which is toxic when inhaled. Oxides of sulfur, generally known as SO.sub.2, are also a problem. Both NOx and SO.sub.2 compounds contribute to the formation of acid rain, which is harmful to plant life, animal life, and property. Mercury, in very small concentrations, has been shown to be highly toxic to humans.
The typical methods of reducing fine particulate matter emissions is by the use of an electrostatic precipitator (ESP) or fabric filter bag houses. The typical methods of reducing SO.sub.2 emissions are wet or dry scrubbers, burning low-sulfur coal, and employing flue gas desulfurization (FGD) apparatuses. Burning low-sulfur coal reduces the particulate collection efficiency of the ESP, and is generally more expensive than ordinary coal. FGD equipment is very expensive to build and operate. The typical method of reducing NOx emissions is the use of special low NOx burners to cool the combustion temperature to a point where the bonds of N.sub.2 present in the combustion air are less likely to be broken. This has the disadvantage of making combustion less efficient and increases particulate emissions. Expensive selective catalytic and non-catalytic reduction systems using ammonia and urea injection have also been tried. These devices are very expensive to purchase and operate. They can also require large amounts of space at the plant site to install. Altogether, current methods for reducing fine particle, SO.sub.2 and NOx emissions can increase the cost of electricity produced at an electric utility by over fifty percent.
Stauffer, in U.S. Pat. No. 4,925,639, that issued on May 15, 1990, disclosed a process for removing NOx from flue gas and making HNO.sub.3 as a useful by-product. The process involved cyclically subjecting the gas to scrubbing with nitric acid and then electrolyzing the dissolved nitric oxide to form more nitric acid. This process has the disadvantage that it only treats one type of pollution.
A few have tried to remove multiple pollutants from a flue gas stream. Plaks et al., in U.S. Pat. No. 5,601,791, that issued on Feb. 11, 1997, discloses a process and apparatus that neutralizes "acid gases" such as SO.sub.2 inside an existing ESP. Plaks et al. spray a neutralizing agent upstream from the ESP collecting plates to collect particulates, neutral salts, and unreacted neutralizing agent. The material collected on the plates is then washed using a spray in the manner of a wet ESP. This process and apparatus does not purposefully create and collect the acids, which are valuable industrial materials. Instead, the resulting effluent is sent to a landfill for disposal.
Sparks et al., in U.S. Pat. No. 4,885,139, that issued on Dec. 5, 1989, discloses a method for removing SO.sub.2 and other "acid gases" from flue gas by a multi-stage ESP within a single housing. In that method, a neutralizing agent is sprayed upstream from the ESP collecting plates, forming neutral salts which dry before being collected by the plates. In this manner SO.sub.2 and particulates are removed from the flue gas. However, like Plaks et al., no effort is made to form H.sub.2 SO.sub.4 from the SO.sub.2, and the effluent must be sent to a landfill for disposal. Nor do either of them refer to the removal of NOx or the formation of HNO.sub.3 in this manner.
The deleterious health effects of these noxious pollutants become better understood as more medical research is completed. As a result, environmental regulations world-wide are being made more stringent. Although mercury emissions from fossil fuel fired boilers are not yet regulated, this is likely to change as research has shown that over 20 percent of mercury emissions in the United States come from coal fired power plants. When the environmental regulations become more stringent, the cost of compliance increases. More expensive pollution control equipment must be purchased and maintained which does not provide any monetary return to the plant owner.
While environmental compliance costs continue to rise, there is a movement toward consolidating ownership of power plants world-wide and increasing competition. As a result, capital expense budgets are often slashed in an effort to keep the cost of producing electricity low. A pollution control process and apparatus that can provide a monetary return to the owner while reducing particulate, NOx, and SO.sub.2 emissions would solve several serious problems at the same time.
To date, a limited number of plants have been able to sell collected particulate matter commercially. Of the gases, only SO.sub.2 has been converted to useful products that can provide a monetary return. It has been used in the manufacture of gypsum and in the recovery of elemental sulfur. Also, dilute acids have been manufactured from exhaust gases by catalytic reactions. These methods are limited, and are not widely used.
For the foregoing reasons, there is a need for a process and apparatus for reducing particulate, NOx, and SO.sub.2 emissions from the combustion of fossil fuel while producing an end product that is commercially useful and eliminating the need to dispose of an environmentally undesirable by-product.